Low-voltage High-resolution Scanning Electron Microscopy Research Articles

Interconnected 3D Framework of CeO2 with High Oxygen Storage Capacity: High-Resolution Scanning Electron Microscopic Observation

CeO2 nanocasting in a mesoporous hard template provides a three-dimensionally (3D) interconnected skeleton framework consisting of CeO2 nanoparticles with particles smaller than 7 nm. Low-voltage high-resolution scanning electron microscopy with Ar-ion-beam cross-sectional polishing revealed numerous CeO2 (∼250 nm) clusters distributed in the SBA-15 mesochannels. Each CeO2 framework consists of CeO2 primary nanocrystals that are mutually and coherently connected. The mesochannel walls of the mesoporous silica might confine the growth of the CeO2 nanocrystals, resulting in a high oxygen storage capacity of the material. A series of cross-sectional images obtained by using damage-free Ar-ion-beam polishing revealed the mechanism of pore filling and CeO2 structuring process: that from the nucleation and growth of the CeO2 in the mesopores to the self-organization of nanocrystals that form multiple nanowires along the mesochannels. Furthermore, self-organized nanowires were arranged periodically and were mutually connected finally to form a negative replica of the ordered mesoporous SBA-15 silica.

Direct Observation of Nano-porous Materials Using Low Voltage High Resolution SEM

Direct Observation of Nano-porous Materials Using Low Voltage High Resolution SEM

Publisher’s Note: “Direct observation and analysis of yolk-shell materials using low-voltage high-resolution scanning electron microscopy: Nanometal-particles encapsulated in metal-oxide, carbon, and polymer” [APL Mater. 2, 113317 (2014)

Publisher’s Note: “Direct observation and analysis of yolk-shell materials using low-voltage high-resolution scanning electron microscopy: Nanometal-particles encapsulated in metal-oxide, carbon, and polymer” [APL Mater. <b>2</b>, 113317 (2014)

Direct observation and analysis of yolk-shell materials using low-voltage high-resolution scanning electron microscopy: Nanometal-particles encapsulated in metal-oxide, carbon, and polymer

Nanometal particles show characteristic features in chemical and physical properties depending on their sizes and shapes. For keeping and further enhancing their features, the particles should be protected from coalescence or degradation. One approach is to encapsulate the nanometal particles inside pores with chemically inert or functional materials, such as carbon, polymer, and metal oxides, which contain mesopores to allow permeation of only chemicals not the nanometal particles. Recently developed low-voltage high-resolution scanning electron microscopy was applied to the study of structural, chemical, and electron state of both nanometal particles and encapsulating materials in yolk-shell materials of Au@C, Ru/Pt@C, Au@TiO2, and Pt@Polymer. Progresses in the following categories were shown for the yolk-shell materials: (i) resolution of topographic image contrast by secondary electrons, of atomic-number contrast by back-scattered electrons, and of elemental mapping by X-ray energy dispersive spectroscopy; (ii) sample preparation for observing internal structures; and (iii) X-ray spectroscopy such as soft X-ray emission spectroscopy. Transmission electron microscopy was also used for characterization of Au@C.

Direct Observation of Plugs and Intrawall Pores in SBA-15 Using Low Voltage High Resolution Scanning Electron Microscopy and the Influence of Solvent Properties on Plug-Formation

Through the use of low voltage high resolution scanning electron microscopy (LV-HRSEM) we have studied the fine details of the intricate pore structure of SBA-15. Intrawall pores and deviations from the ideal and uniform cylindrical pores are clearly observed, and we report for the first time the direct observation of “plugs” in the pores. N2-sorption measurements confirm their existence. LV-HRSEM provides an opportunity to quantify the frequency of occurrence of plugs within the pore structure. The rate of mesophase formation, followed with in situ small angle X-ray scattering (SAXS) under different solvent conditions, is shown to have a significant influence on the development of plugs and how frequently they occur. We suggest a mechanism explaining the existence of the plugs, providing means for a better understanding and control over material properties.

Holographic Polymer Dispersed Liquid Crystals: Effect of Partial Matrix Fluorination on Electro-Optical and Morphological Properties

ABSTRACTHolographic polymer dispersed liquid crystal (H-PDLC) films with partially fluorinated matrices were investigated. Electro-optical and morphological studies revealed that fluorinated composites were substantially different from non-fluorinated analogues. The addition of a fluorinated monofunctional acrylate monomer to a pentaacrylate-derived polymer matrix resulted in improved diffraction efficiency. These findings suggest that the partial fluorination of the host polymer decreases the compatibility between the matrix and liquid crystal phase. Morphological differences between fluorinated films and non-fluorinated control specimens were verified using low-voltage, high-resolution scanning electron microscopy (LVHRSEM).

Novel Xerogel Catalyst Materials for Hydrogenation Reactions and the Role of Atomic Scale Interfaces

We are developing a new family of heterogeneous catalysts for hydrogenation catalysis. Catalyst synthesis is accomplished using colloidal polymerization chemistry which produce high surface area xerogel catalysts. These xerogels have been synthesized by one-step sol gel chemistry. These catalysts contain ruthenium and modifiers such as gold occluded or incorporated in a titanium oxide matrix. The materials, especially the modified systems exhibit favorable performance in microreactor evaluations for hydrogenation reactions and exhibit high activities. Nanostructural studies have revealed that the materials contain dispersed catalyst clusters which are desirable microstructures for the catalysis since the majority of the atoms are exposed to catalysis and are potentially active sites.The composition and atomic structure of the xerogel catalysts containing ruthenium and other metals have been examined using our in-house developments of environmental high resolution electron microscopy (EHREM) the atomic scale [1-3] and low voltage high resolution SEM (LVSEM)[4] methods.

Microstructural Characterization of Bombyx mori Silk Fibers

The microstructure of Bombyx mori silk fibers before and after degumming was examined by transmission electron microscopy (TEM), selected area electron diffraction (SAED), wide-angle X-ray scattering (WAXS), and low-voltage high-resolution scanning electron microscopy (LVHRSEM). LVHRSEM micrographs of the neat cocoon revealed a network of pairs of twisting filaments. After degumming, there were only individual filaments showing a surface texture consistent with an oriented fibrillar structure in the fiber interior. WAXS patterns confirmed the oriented β-sheet crystal structure common to silkworm and spider (dragline) silks. Low-dose SAED results were consistent with the WAXS data and revealed that the crystallographic texture did not vary significantly across the fiber diameter. TEM observations of individual microtomed fibers indicated a nominally triangular cross section and a 0.5−2 μm sericin coating. After degumming to remove the sericin, a banded feature was revealed with a characteristic spacing of nominally 600 nm along the fiber axis. These bands were oriented in a roughly parabolic or V-shape pointing along one axis within a given fiber. We hypothesize that this orientation was induced by the extrusion and drawing during the spinning process. Equatorial dark field (DF) images revealed that axial and lateral sizes of the β-sheet crystallites in silk fibroin ranged from 20 to 170 nm and from 1 to 24 nm, respectively. Crazes developed in the degummed silk fiber parallel to the fiber direction. The formation of these crazes suggests that there are significant lateral interactions between microfibrils in silk fibers.

Use of Low Voltage High Resolution SEM (LVHRSEM) to Reveal Architectural Organization of Bone Mineral

Load-bearing tissues in different organ systems display a wide variety of mechanical properties, yet are composed of only a few different molecules, with collagen as the principal structural protein. In bone, a mineral phase comprising carbonate-apatite crystallites confers compressive and torsional strength. The arrangement of collagen and apatite into three-dimensional hierarchical composite structures enables the tissue to bear high loads while maintaining flexibility. Moreover, bone serves as the body’s chemical storehouse for non-collagenous proteins and smaller molecular weight anions and cations such as calcium.An understanding of the three-dimensional structure of the apatite phase at the nanometer and micrometer levels can provide insights into the properties of bone, and serve as the guide for the development of bone substitute materials. Transmission electron microscopy (TEM) has been valuable in revealing certain ultrastructural features of the mineral. However, it has not yet been possible to obtain three-dimensional reconstructions of the morphology and distribution of the apatite phase of bone using this method.

Low-voltage, high-resolution scanning electron microscopy: a new characterization technique for polymer morphology

Low-voltage, high-resolution scanning electron microscopy (LVHRSEM) is a promising new technique for polymer morphological characterization, and provides complementary data to transmission electron microscopy, X-ray scattering and the scanning probe microscopies. Practically, lateral resolution on the order of 50 Å at 1.0 keV accelerating voltage can be obtained in polymer samples. The utility of LVHRSEM is demonstrated for diverse polymer systems, both amorphous and crystalline, with structurally interesting features on a 50–1000 Å length scale. The advantages and disadvantages of LVHRSEM as a polymer characterization technique are investigated, as well as the effects of beam—sample interactions, radiation damage and contamination specific to polymers examined at low voltage.

Cell-surface receptors and proteins on platelet membranes imaged by scanning force microscopy using immunogold contrast enhancement

High resolution scanning force microscope (SFM) images of fibrinogen-exposed platelet membranes are presented. Using ultrasharp carbon tips, we are able to obtain submolecular scale resolution of membrane surface features. Corroboration of SFM results is achieved using low voltage, high resolution scanning electron microscopy (LVHRSEM) to image the same protein molecule that is seen in the SFM. We obtain accurate height dimensions by SFM complemented by accurate lateral dimensions obtained by LVHRSEM. The use of 14- and 5-nm gold labels to identify specific membrane-bound biomolecules and to provide contrast enhancement with the SFM is explored as a useful adjunct to observation of unlabeled material. It is shown that the labels are useful for locating specific protein molecules on platelet membrane surfaces and for assessing the distribution of these molecules using the SFM. Fourteen nm labels are shown to be visible over the membrane corrugation, whereas 5-nm labels appear difficult to resolve using the present SFM instrumental configuration. When using the 5-nm labels, collateral use of LVHRSEM allows one to examine SFM images at submolecular resolution and associate function with the structures imaged after the SFM experiment is completed.

Atomic force microscopy and high-resolution scanning electron microscopy study of the banded surface morphology of hydroxypropyl cellulose thin films

Low voltage high resolution SEM and atomic force microscopy (AFM) studies both indicate that a sinusoidal fibrillar morphology is associated with the banded texture observed in hydroxypropylcellulose thin films. The AFM studies clearly demonstrate that the fibrillar trajectory deviates from lying on the plane of shear, and for shear bands with 6 μm wavelength, this deviation was found to be ∼ 4-10°

Cholesteric liquid crystals: Image contrast in the TEM

Abstract Bright-field image contrast of thermotropic cholesteric liquid crystalline materials in the transmission electron microscope (TEM) is investigated. Possible sources of contrast for these systems are discussed in terms of their molecular anisotropy. A cholesteric side-chain liquid crystalline compound was investigated with TEM, low voltage high resolution scanning electron microscopy (LVHRSEM), and atomic force microscopy (AFM) to determine the origin of the strong contrast observed in these systems using bright-field transmission electron microscopy. Initial contrast of thin microtomed sections, as viewed with TEM low dose techniques and an image intensifier, was much weaker than observed under normal viewing conditions. The periodic steady state contrast typically observed for these materials is the result of beam damage. Furthermore, the surface of microtomed samples (parallel to the cholesteric helical axis) is corrugated with a periodicity of 1/2 the pitch due to a preferred fracture path in ...

Low-voltage high-resolution SEM: A valuable resource for the polymer morphologist

The elucidation of polymer morphology has historically been accomplished using optical microscopy (OM), conventional transmission electron microscopy (TEM), X-ray scattering and more recently for crystalline polymers, high resolution TEM (HRTEM). Significant work using scanning electron microscopy (SEM) to study synthetic polymers began in the late 1960's, but for twenty years after these first studies, the smallest features imaged by SEM in polymer samples have typically been 1-10 μm. The evolution of the low voltage high resolution SEM (LVHRSEM) now provides polymer scientists with the opportunity to image structures topographically on the order of 50-100 Å, a level of microstructure important in basic polymer physics research. The potential for improved understanding of polymer morphology is significant, especially when HRSEM data is combined with information from other complementary techniques such as TEM, X-ray scattering, and the scanning probe microscopies (SPM, AFM, STM).The advantages of LVHRSEM over conventional thermionic source SEM as a polymer characterization technique include: i) low keV operation reduces or eliminates charging in uncoated insulating polymers, ii) the high brightness, low energy spread, small spot size field emission gun allows for operation at low keV without significant loss of resolution (practically, resolution on the order of 50 Å at 1.0 keV in a polymer sample is attainable), iii) imaging of surface topography is improved at low keV as the beam/sample interaction volume is smaller and closer to the surface, resulting in more secondary electrons being generated within the secondary electron escape depth, and iv) the immersion lens system improves the signal to noise ratio by decreasing the noise component in the secondary electron signal.

AFM, LVHRSEM, and cross-sectional TEM of the near-surface morphology of diblock copolymers

Application of block copolymers as adhesives and biomedical devices requires an understanding of the surface composition and morphology of these materials. Microscopy techniques offer excellent depth and lateral resolution and therefore can provide direct microstructural and compositional surface data. Previous workers frequently used only a single microscopy technique to investigate the influence of an external surface constraint on the morphology. The implementation of several different microscopy techniques, each with its own strengths, affords a very complete characterization of the surface composition and morphology of self-assembling diblock copolymers.The investigation of solvent-cast and annealed thick (1 mm) films of two poly(styrene-b-butadiene) (SB) diblock copolymers, which usually self-assemble to form alternating poly(styrene) (PS) and poly (butadiene) (PB) lamellar microdomains, by three complementary microscopy techniques: cross-sectional transmission electron microscopy (CSTEM), low-voltage high-resolution scanning electron microscopy (LVHRSEM), and atomic force microscopy (AFM) is discussed. For CSTEM and LVHRSEM imaging, contrast between the PS and PB phases was enhanced by preferential staining of the PB phase with the vapors of a 4% aqueous osmium-tetroxide (OsO4) solution for 24 hours.

Low-voltage high-resolution SEM investigations of novel macromolecular liquid crystals

The ability to visualize detailed 3-dimensional surface topology with SEM at low voltage and high resolution holds profound promise for analyzing liquid crystal textures, both in polymers and other macromolecular forms. Director textures, domain boundaries, and defects such as inversion walls, disclinations and dislocations can now be easily visualized with this technique. Studies concerning the effects of shear flow and magnetic fields on these defects are currently under way.Resolution of 4.0 nm at 1.0 keV is now possible with commercial SEM's, which incorporate the latest advances in lens design optimized for low voltage operation, and use high brightness, low energy spread field emission electron guns. The low energy spread of the field emission gun reduces chromatic aberration effects and facilitates successful operation at low keV. Low voltage operation provides dramatically improved image contrast due to the smaller beam/sample interaction volume and also greatly reduces sample charging artifacts. By operating at near crossover conditions, the need for coating nonconducting specimens with a conducting layer of metal or carbon is greatly reduced or eliminated.

Application of low-voltage, high-resolution SEM in the study of complex intracellular structures

Cellular architecture is a dynamic web of complex macromolecular assemblies accomplishing the diverse functions of a cell. Conventional electron microscopy on ultrathin sections or negatively stained preparations can provide little information on more extended three dimensional assemblies. High voltage and intermediate voltage TEM provide high resolution in much thicker specimens but are limited by problems of contrast and overlap of structures. In recent years new SEMs have become available that provide the high topographic contrast and three dimensionality of SEM at a resolution comparable to conventional TEM. I have used the low voltage high resolution SEM Hitachi S-900 at the Madison IMR and shall show some examples that illustrate the usefulness of LVSEM in structural cell biology. Most striking is the new information obtained about the nuclear pore complex (NPC). This structure is extremely important in controlling the selective and unidirectional transport of large molecules into and out of the nucleus.

Correlative video enhanced differential interference contrast light microscopy (VDIC)-LM), low-voltage high-resolution SEM (LV-HR-SEM), and high-voltage electron microscopy (HVEM) in the observation of gold-labelled surface antigens and the subjacent cytoskeletal matrix

The development of video-enhanced light microscopy (LM) as well as associated image processing and analysis have significantly broadened the scope of investigations which can be undertaken using (LM). Interference/polarization based microscopies can provide high resolution and higher levels of “detectability” especially in unstained living systems. Confocal light microscopy also holds the promise of further improvements in resolution, fluorescence studies, and 3 dimensional reconstruction. Video technology now provides, among other things, a means to detect differences in contrast difficult to detect with the human eye; furthermore, computerized image capture, processing, and analysis can be used to enhance features of interest, average images, subtract background, and provide a quantitative basis to studies of cells, cell features, cell labelling, and so forth. Improvements in video technology, image capture, and cost-effective computer image analysis/processing have contributed to the utility and potential of the various interference and confocal microscopic instrumentation.Electron microscopic technology has made advances as well. Microprocessor control and improved design have contributed to high resolution SEMs which have imaging capability at the molecular level and can operate at a range of accelerating voltages starting at 1KV. Improvements have also been seen in the HVEM and IVEM transmission instruments. As a whole, these advances in LM and EM microscopic technology provide the biologist with an array of information on structure, composition, and function which can be obtained from a single specimen. Corrrelative light microscopic analysis permits examination of living specimens and is critical where the “history” of a cell, cellular components, or labels needs to be known up to the time of chemical or physical fixation. Features such as cytoskeletal elements or gold label as small as 0.01 μm, well below the 0.2 μm limits of LM resolution, can be “detected” and their movement followed by VDIC-LM. Appropriate identification and preparation can then lead to the examination of surface detail and surface label with stereo LV-HR-SEM. Increasing the KV in the HR-SEM while viewing uncoated or thinly coated specimens can provide information from beneath the surface as well as increasing Z contrast so that positive identification of surface and subsurface colloidal gold or other heavy metal labelled/stained material is possible. Further examination of the same cells using stereo HVEM or IVEM provides information on internal ultrastructure and on the relationship of labelled material to cytoskeletal or organellar distribution, A wide variety of investigations can benefit from this correlative approach and a number of instrumentational configurations and preparative pathways can be tailored for the particular study. For a surprisingly small investment in time and technique, it is often possible to clear ambiguities or questions that arise when a finding is presented in the context of only one modality.

Low-voltage high-resolution Scanning Electron Microscopy of semiconductors

The advent of scanning electron microscopes (SEMs) with reliable, high performance field emission guns (FEG) has afforded many opportunities to obtain new information at low voltages not available at higher voltages in traditional SEMs equipped with tungsten hairpin or LaB6 filaments. The FEG SEMs are able to operate at low voltages with both high brightness and high resolution (HR) due to the small source size and low energy spread of the beam. Resolution of 4 nm down to 1.5 nm are routinely possible in the energy range from 1 to 5 keV along with standard image recording times of 1 to 2 minutes. The low voltage capabilities have allowed insulating materials, such as polymers, composites, and ceramics to be imaged at high resolutions at energies below the second crossover, usually around 1 to 2 keV, without experiencing image artifacts from negative surface charging normally found in uncoated insulators at higher operating voltages.

Low-voltage high-resolution SEM of biological samples

There are three reasons for the recent upsurge of interest in using the SEM at a beam voltage (Vo) around 1 kV (LVSEM). The most common one arises from applications in the semiconductor industry and emphasizes the reduction in charging artifacts and in subsurface beam damage obtainable at low VoThe second reason derives from the belief that, given instrumentation capable of producing a sufficiently small probe, the increased contrast in the topographic component of the secondary electron signal will permit an important improvement in real topographic spatial resolution.The third reason is that recent developments in instrumentation have shown that by coupling a cold field-emission source with a short focal length lens, it is indeed possible to obtain small probe diameters at low voltage. Although they are considerably larger than probes obtained at higher voltage, they are nonetheless smaller than the smallest topographical feature yet imaged in the secondary electron (SE) mode. (Fig 1)